The present invention relates generally to high-voltage phasing meters and, in particular, to long-range wireless phasing meters that use communications links to transfer data.
Three-phase high-voltage distribution and transmission lines consist of three energized conductors and a fourth, neutral or “ground” conductor. The three energized conductors each carry an electrical voltage that varies in magnitude at the same frequency but the phase of the voltage carried by each conductor is displaced by a phase angle of 120 degrees. The conductors carrying these three differently-phased voltages are generally labeled as the A, B, C or 1, 2, 3 conductors, or equivalent depending on the utility, to tell them apart. In the simplest arrangement, the first phase, or reference phase, is arbitrarily designated to be 0 degrees, making the next phase 120 degrees displaced from the first and the last phase 240 degrees displaced from the first.
When two sets of high voltage distribution and transmission lines are to be connected, the phases of each line must match. In total, there are six possible ways to attach any two sets of three conductors. Each of these six different connections will result in a different outcome for the device being powered, an outcome that may be significant. Incorrectly-wired three-phase transformer banks, consisting of three individual transformers, for example, can produce phase angles between 0 and 360 degrees in 30 degree steps. Accordingly, phase identification prior to connecting conductors is important to those who maintain high voltage distribution and transmission systems.
Unfortunately, the phase identity of individual conductors may get lost in overhead distribution and transmission systems. In underground electrical systems, which may extend for many miles, the phase identification ascribed to individual conductors is not always correct. Unauthorized digging or trenching up of an underground electrical system, which is a common occurrence, may sever conductors and result in loss of phase identification. Also, natural disasters such as accidents, hurricanes, tornadoes, forest fires, high winds, snow, ice, earthquakes, floods, etc. may result in loss of phase identification in above-ground and even in underground transmission systems. Mapping, phase tagging and verification of system records for both above-ground and underground electrical systems require accurate phase identification.
Determining the time varying voltages of two conductors is part of the measurement but when the conductors are far enough apart, the fact that they are separated will introduce errors into the comparison of the two measurements. Eliminating, correcting or avoiding those errors is vital to correctly connecting separated conductors. Achieving these objectives with a minimum use of communications bandwidth and a minimum amount of data transfer may have advantages.
Measuring the phase difference between the voltages on electrical conductors per se is known. One system is disclosed in U.S. Pat. No. 6,642,700 issued to Slade et al and assigned to Avistar Inc. This system identifies phase angles of electrical conductors at remote locations by measuring the time delay between an external clock source and a zero crossing of the wave form. A time tag is associated with that time delay and transmitted over a full-duplex communications link between a field unit and a reference unit. At the reference unit, the phase angle is calculated and displayed. The Avistar system uses the global positioning satellite (GPS) system as its external clock for determining the time delay. In order for this system to operate in real time, it requires either a half-duplex or full-duplex, full-time communications link of relatively high speed to transfer all of the time tag and overhead voltage information.
Another phase angle measurement system is described in U.S. Pat. Nos. 6,734,658 and 7,109,699, issued to the present inventor. In this system, a signal, corrected for capacitive charging currents, is obtained by a master probe measuring the voltage carried by a conductor in the field. The phase of a signal from the master probe is compared to the phase of another signal transmitted wirelessly and in full duplex from a supplemental probe that measures a reference voltage. The phase difference is displayed by the master probe. This system compensates for the phase shift introduced when a signal is sent from one probe to the other. The transmitted voltage signal is encoded onto a carrier wave by modulating that wave with the voltage information itself. This system also requires the use of a full-time half- or full-duplex communications channel of relatively high speed.
A third system is described in U.S. Pat. Nos. 6,734,658, 7,808,228, 8,283,910, and 8,283,911, issued to the present inventor. In this system the phase of a voltage carried by a reference conductor is measured by a reference probe and compared to the phase of a precision 60 Hz wave form generated from a GPS receiver signal. The phase difference between these two waves, in the form of a nine-bit data signal is transmitted over a distance, perhaps miles, to a receiver that decodes the data signal and uses another precision 60 Hz wave form generated by another GPS receiver to re-create a surrogate wave identical to the original reference voltage. This surrogate wave is forwarded to a (nearby) meter probe that is measuring the voltage on a field conductor. The meter probe can then compare the two waves to determine the phase angle difference between them. This system is an improvement over the previous two systems relating to the communications requirements since it only requires a low-speed, simplex data channel.
These three prior art systems thus use different ways of obtaining signals that represent the phases of the voltages carried by of the reference and field conductors for comparison and have different communications requirements. The Avistar system compares time tags of the field and reference voltage signals, wherein the time tag of each is the difference between a GPS time and the zero crossing time of the alternating voltage, to determine the phase difference between the two time-varying voltages.
The first Bierer system compares the phase of the reference conductor voltage to that of the field conductor voltage directly but compensates for the phase shift of the transmitted reference voltage resulting from the transmission distance to the master probe measuring the field conductor voltage.
The second Bierer system determines the phase angle between a reference conductor voltage and a precision 60 Hz wave form generated from a GPS signal. This phase angle is transmitted to a distant receiver where a surrogate of the original reference conductor voltage is being re-created. The surrogate is then compared to the phase of the voltage carried by a field conductor.
There remains, however, a need for high voltage phasing voltmeter that is accurate, easy to read and useable when the high voltage distribution or transmission lines are separated by many miles and that does not depend on high quality data communications channels.